PTFE is a well-known and highly useful polymeric material typically employed in various applications involving exposures to high temperatures and/or highly corrosive environments. However, certain mechanical properties of PTFE limit its usefulness. One such limiting property is resistance to fracture upon impact, so-called impact resistance. A widely accepted method for measuring the impact resistance of polymers is the "notched Izod" test, ASTM D256-90.
Commercially available PTFE is a thermoplastic polymer of unusually high molecular weight which cannot be fabricated using conventional techniques used with thermoplastic polymers of lower molecular weight, such as melt extrusion or injection molding. Instead, powder processing methods are typically applied to form shaped articles from PTFE. These powder processing methods include direct forming of shaped articles by cold compaction of PTFE powder, followed by "free sintering" at temperatures at or above the melting point of the virgin polymer; ram extrusion; paste extrusion; and high isostatic pressure processing. Most commonly, billets are formed by cold compaction and free sintering, followed by machining into the final shaped article. Shaped articles of PTFE, fabricated by conventional processing, exhibit notched Izod impact values of up to about 3.5 ft-lbs/inch of notch.
A processing method known as "hot compression molding" (HCM), involving simultaneous application of pressure and elevated temperature, without introducing significant orientation into the article, is commonly used for molding thermoplastic polymers other than PTFE homopolymer. HCM has not, however, been disclosed hitherto for fabricating PTFE. U.S. Pat. No. 4,824,898 discloses PTFE articles having Izod impact resistance of 19-60 kg-cm/cm of notch (3.5-11.0 ft-lb/in of notch) prepared by forming a "preform" by conventional cold compression and sintering at temperatures above 327.degree. C. The preform is then converted into a highly oriented article such as sheeting by what is termed compression molding or extrusion molding at a temperature in the range of 150.degree. C. to 340.degree. C. This molding procedure, which differs markedly from HCM, involves extrusion through dies under plug flow conditions to introduce biaxial or multiaxial orientation into the article. The oriented article is then partly relaxed by heat-shrinking at 150.degree. C. to 300.degree. C. Articles produced by HCM are not oriented.